index.js•1.52 kB
'use strict';
const Queue = require('yocto-queue');
const pLimit = concurrency => {
if (!((Number.isInteger(concurrency) || concurrency === Infinity) && concurrency > 0)) {
throw new TypeError('Expected `concurrency` to be a number from 1 and up');
}
const queue = new Queue();
let activeCount = 0;
const next = () => {
activeCount--;
if (queue.size > 0) {
queue.dequeue()();
}
};
const run = async (fn, resolve, ...args) => {
activeCount++;
const result = (async () => fn(...args))();
resolve(result);
try {
await result;
} catch {}
next();
};
const enqueue = (fn, resolve, ...args) => {
queue.enqueue(run.bind(null, fn, resolve, ...args));
(async () => {
// This function needs to wait until the next microtask before comparing
// `activeCount` to `concurrency`, because `activeCount` is updated asynchronously
// when the run function is dequeued and called. The comparison in the if-statement
// needs to happen asynchronously as well to get an up-to-date value for `activeCount`.
await Promise.resolve();
if (activeCount < concurrency && queue.size > 0) {
queue.dequeue()();
}
})();
};
const generator = (fn, ...args) => new Promise(resolve => {
enqueue(fn, resolve, ...args);
});
Object.defineProperties(generator, {
activeCount: {
get: () => activeCount
},
pendingCount: {
get: () => queue.size
},
clearQueue: {
value: () => {
queue.clear();
}
}
});
return generator;
};
module.exports = pLimit;